The human platelet alloantigen (HPA) system is critical for the diagnosis of neonatal alloimmune thrombocytopenic purpura (NAITP), a severe condition caused by maternal alloantibodies against fetal platelet antigens inherited from the father. These antibodies can cross the placenta and destroy fetal platelets, potentially leading to cerebral bleeding in the neonate.

There are five HPA systems, with HPA-1 being the most prevalent (75–85% of cases). The HPA-1 system is biallelic, with alleles HPA-1a and HPA-1b determined by thymidine or cytosine at base 196, resulting in a leucine-to-proline substitution at position 33 of the protein.

Mothers who are HPA-1a negative carrying an HPA-1a positive fetus are at risk of developing anti-HPA-1a antibodies, which can cause neonatal alloimmune thrombocytopenia.

• Clotted samples
• Specimens not collected in EDTA tubes
• Frozen specimens

Polymerase chain reaction (PCR) with reverse hybridization.

The results are reported as follows:

• Positive (Normal)  - Represents HPA-1a/a
• Heterozygous - Represents HPA-1a/b
• Negative - Represents HPA-1b/b

• Forsberg et al. Transfusion 1995;35:241-246.
• Ouwehand G et al. Arch Dis Fetal Neonatal Ed 2000;82:F173-F175.

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